1. Field of the Invention
The present invention relates to cleaning of materials or parts using pressurized water where the nozzle or banks of nozzles are fixed with respect to a moving target or product.
2. Description of the Prior Art
Surface cleanliness, a key parameter in materials processing and manufacturing, significantly affects the quality of a product. Cleaning in the manufacturing environment has traditionally been carried out by many methods, but these generally break down into two categories: chemical and mechanical. Chemical methods have been popular in many industries, especially heavy industries, such as primary metals production, because of the thoroughness of the cleaning, high quality, high productivity and the low cost. Typical chemicals used in such cleaning processes are: water, acids, soaps, chlorofluorocarbon (CFC's), chlorinated hydrocarbons, aromatic hydrocarbons, and aliphatic hydrocarbons. Mechanical processes are typically used where the required surface quality is lower, or chemical processes are less convenient or less effective. Typical mechanical cleaning methods are: grit blasting, shot blasting, grinding, brushing and milling. The use of pressurized water for cleaning is a hybrid of the chemical process and the mechanical process. Specifically, water is in-itself a solvent and when sprayed at high pressures, it acts as an abrasive.
The chemical cleaning methods, while still quite popular, have been waning due to environmental and health concerns. The Clean Air Act Amendments of 1990, as well as other environmental legislation, have reduced the usage of some of the most effective chemicals, such as the volatile organic compounds (VOC's) and phosphate based detergents. The effect has been to send industry looking for the best alternative technologies. Currently used chemical cleaning methods, such as acid pickling of steel, tend to generate a vast quantity of waste that must be disposed of or recycled. And so, the advantages of superior quality and high productivity for chemical methods may soon be lost due to the overwhelming costs of environmental control and waste disposal.
Pressurized water has been used in hot metal production lines. Specifically, during reheating of steel slabs and ingots for hot rolling, the metal reacts with the oxygen in the air to form a thick oxide scale. The scale, formed prior to hot rolling, is referred to as "primary scale". This flaky, porous scale is relatively thick and friable. Typically, it is 0.040-0.050 inches thick. Primary scale is removed prior to hot rolling by a pressurized water descaler operating at pressures less than 3500 psi. The water exits the nozzles in a fan fashion. The nozzles are positioned at a distance, on the order of 6-12 inches, from the surface of the metal. Dispensed water contacts the primary scale, which tends to be somewhat exfoliated, and lifts it away from the metal. Water is also trapped in porous pockets in the scale. At these temperatures, on the order of 1600 degrees Fahrenheit, steam forms quickly and the scale is also dislodged by the rapidly expanding steam.
During cooling of the hot rolled steel, the oxygen in the air again reacts with the metal to form a much thinner and tighter oxide coating. This is referred to as "secondary" scale or post hot rolling scale. The secondary scale is on the order of 0.005 inches thick or less and is dense and uniform in nature. The above-described pressurized water system cannot remove the secondary scale. Therefore, abrasion and/or chemical methods such as pickling must be used. Pressurized water, i.e., water maintained at pressure above 20,000 psi (pounds per square inch) has been used for 10-20 years in applications, such as rock drilling, stripping paint from bridges, metal cutting, cutting of fiber glass circuit boards, and cutting of lumber. More recently, pressurized water has been adapted to more refined applications, such as robotic stripping of paint from airplanes and ships, cleaning electronic circuit boards, CNC machining, cleaning and near net shape machining of metal and ceramic parts. In all these instances the work piece is held stationary and the nozzle, which supplies the high pressure water stream, moves relative to the target.
Work has been carried out by Dr. David Summers at the University of Missouri to develop efficient nozzles for hand-held wands. These hand-held systems have been used at pressures up to 60,000 psi and a flow rate of 1-2 gpm (gallons per minute). Work has also been carried out by the NASA Marshal Space Flight Center (MSFC) where ultra high pressure water has been adopted to robotically move the high pressure nozzle over a stationary object (a reusable rocket booster) to remove left over fuel. Similarly, the Air Force has been testing a robotic system to remove paint from airplanes. MSFC has also been experimenting with the injection of a solid abrasive into the high pressure stream to increase the efficiency of the process. The abrasive currently used is made of baking soda (sodium bicarbonate) and the process leaves solids that must be disposed of. A problem with abrasive type system is that the abrasive must be disposed of offsite. This is a costly endeavor.
It is an object of the present invention to provide an improved cleaning method for materials processing with increased productivity.
It is further an object of the present invention to provide a substrate cleaning method that is faster than the prior state of the art, results in a more uniform surface quality than the prior state of the art and minimizes waste.